1. Technical Field
The present invention relates generally to millimeter wave energy emissions and, in particular, to the transmission of millimeter wave energy emissions by a beacon and the passive reception of millimeter wave energy emissions by a receiver.
2. Description of the Related Art
All materials at temperatures above absolute zero (0xc2x0 K) radiate electromagnetic energy as a function of their temperatures. For terrestrial objects, this radiation, often referred to as xe2x80x9cblackbody radiation,xe2x80x9d peaks in the infrared band at wavelengths of 5-25 microns and falls off rapidly at shorter wavelengths through the visible, ultraviolet, x-ray, gamma ray and cosmic ray spectrums. At longer wavelengths, through the far-infrared, submillimeter wave, and millimeter wave (MMW) spectrums, ambient radiation again falls off, but much less rapidly. Passive imaging at these wavelengths from ambient blackbody radiation is possible if radiation levels are sufficient for available detectors to detect and process.
As shown in FIG. 1, in the terrestrial environment, atmospheric gases and compounds such as water, carbon dioxide, and oxygen absorb blackbody radiation in much of the MMW spectrum to such an extent that observable signal strength is below detectable levels. However, as indicated at reference numerals 10 and 12, blackbody radiation at certain frequencies within the MMW spectrum can be detected with currently available MMW detectors due to decreased atmospheric absorption. In particular, observation windows for atmospheric blackbody radiation are present at approximately 35, 95, 140, and 220 GHz.
MMW detection and imaging are similar to conventional infrared imaging in that the received signal strength observed utilizing each technology is a function of the temperature of the elements composing the observed scene. Thus, the observation of an object through detection of its blackbody radiation depends upon the existence of a sufficient temperature contrast with the surrounding environment. As depicted in FIG. 2, terrestrial MMW temperatures range from approximately 30xc2x0 K to 300xc2x0 K, with the coldest temperatures within a cone of approximately 40xc2x0 centered on the zenith and the warmer temperatures produced by the surrounding terrestrial environment. This large temperature differential of 270xc2x0 K provides sufficiently high contrast for MMW detection and imaging. Although there is some solar heating, it should be noted that this temperature differential is relatively independent of sunlight and therefore present during both day and night observations.
Currently, several companies, such as TRW of Redondo Beach, Calif. and Thermo Trex Corporation of San Diego, Calif., manufacture passive MMW cameras for applications such as all weather aircraft take-off and landing, airborne surveillance and reconnaissance through fog, cloud, dust, smoke and camouflage, concealed weapon detection, vehicle navigation, and other applications where enhanced vision is desired. Although promising because of the low attenuation of MMW versus higher frequency radiation (e.g., IR), MMW camera technologies are subject to inherent limitations in image resolution and angular resolution due to the relatively longer wavelengths of MMW radiation. Consequently, the present invention recognizes that a need exists for an improved method and system for determining the presence and location of an object within a field of interest by detecting its MMW signature.
In view of the foregoing and additional needs recognized by the present invention, object detection from millimeter wave emissions is enhanced by incorporating a millimeter wave beacon within an object of interest. In accordance with the present invention, the millimeter wave beacon includes a millimeter wave transmitter that generates at least one signal, and a plurality of antennas, coupled to the millimeter wave transmitter, that each emit millimeter wave radiation in response to the signal(s) generated by the millimeter wave transmitter. In one embodiment, the antennas are constructed and oriented to produce an overlapping signal field in which millimeter wave radiation emitted from multiple of the antennas can be detected. The millimeter wave radiation can be pulsed to encode various information of interest. Determination of the direction of approach to the beacon is facilitated by multiple antennas transmitting signals having different pulse encodings or different frequencies.
Additional objects, features, and advantages of the present invention will become apparent from the following detailed written description.